Continuous Coverage V.F.O. for H.F.

Transcription

1 Continuous Coverage V.F.O. for H.F. Introduction This project arises from the need to home-build a valid tuning control for a multi band transceiver. It consists of a partial syntesis V.F.O. that fits to single conversion equipments with an I.F. stage nearby 9 MHz. The circuit can cover the whole H.F. band from 3.5 to 30 Mhz (i.e to 39 Mhz output). This device has been developed through several experiments based on PLL and crystal conversion circuits, and I think it may represent an acceptable compromise between the simplicity ( but not enough to be regarded as an elementary job ) and the performance. Consider that some equipment is necessary for the alignment : an R.F. generator and a frequency meter are a must, but the availability of an oscilloscope makes the job easier (specially in case of troubles) How it is made It consists of two phisically separated units : 1) V.F.O. module 1

2 Part List of VFO unit R1 : 1 KΩ C4 : 100 pf C24 : 15 pf trim L1 : 24 turns/0.8 mm wire/13 mm diam µh R2 : 100 Ω C5 : 47 pf C25 : 22 pf L2 : 9 turns/0.5 mm wire/t44-2 core 0.42 µh R3 : 47 KΩ C6 : 100 pf C26 : 1.5 pf L3 : 2 turns/0.5 mm wire on L4 R4 : 100 KΩ C7 : 6.8 pf C27 : 15 pf trim L4 : 9 turns/0.5 mm wire/t44-6 core 0.34 µh R5 : 220 Ω C8 : 60 pf var C28 : 22 pf L5 : 9 turns/0.5 mm wire/t44-6 core 0.34 µh R6 : 68 KΩ C9 : 120 pf N150 C29 : 47 pf R7 : 68 KΩ C10 : 47 nf C30 : 100 pf R8 : 100 Ω : 100 nf C31 : 2.2 pf R9 : 1 KΩ C12 : 220 pf C32 : 220 pf R10 : 1 KΩ C13 : 68 pf T1 : 2N3819 R11 : 100 KΩ C14 : 6.8 pf T2 : 2N2222 R12 : 100 KΩ C15 : 220 pf T3 : 2N2222 R13 : 390 Ω C16 : 220 pf T4 : BF960 R14 : 220 KΩ C17 : 47 nf T5 : 2N2222 R15 : 820 Ω C18 : 47 nf T6 : 2N2222 R16 : 100 KΩ C19 : 6.8 pf U1 : 7810 R17 : 220 Ω C20 : 220 pf D1 : 1N4148 C1 : 47 nf C21 : 40 pf trim DV1 : BB205 C2 : 47 nf C22 : 47 nf X1 : 18 MHz C3 : 47 nf C23 : 470 pf is composed, in my arrangement, by three little PCBs (delimited by discontinous line) and the variable capacitor. The three boards are overlapped to form a wafer and the overall cabinet dimension depends essentially on the capacitor size. This unit can be located behind the front panel of the rig 2) PLL Module composed by the VCO PCB 2

3 Part List of VCO unit R1 : 680 Ω R21 : 820 Ω C16 : 10 nf D1 : 1N4148 R2 : 1.2 KΩ R22 : 56 KΩ C17 : 10 nf D2 : 1N4148 R3 : 100 KΩ R23 : 270 Ω C18 : 2.2 pf DV1 : MVAM115/BB112 R4 : 330 Ω R24 : 270 Ω C19 : 68 pf DV2 : MVAM115/BB112 R5 : 56 KΩ R25 : 27 Ω C20 : 10 nf DV3 : MVAM115/BB112 R6 : 1.8 KΩ C1 : 10 nf C21 : 150 pf DV4 : MVAM115/BB112 R7 : 47 KΩ C2 : 22 pf C22 : 1 nf DZ1 : 6.8 V ½ W R8 : 330 Ω C3 : 6.8 pf C23 : 1 nf L1 : 7 turns/0.8 mm wire/5 mm plastic support R9 : 1 KΩ C4 : 6.8 pf C24 : 120 pf with ferrite core/ Lmin 0.18 µh Lmax 0.48 µh R10 : 33 KΩ C5 : 68 pf C25 : 1 nf L2 : 12 turns/0.5 mm wire/5 mm plastic support R11 : 56 KΩ C6 : 330 pf T1 : BF324 with ferrite core/ Lmin 0.52 µh Lmax 1.3 µh R12 : 680 Ω C7 : 10 nf T2 : BF244 L3 : 5 bifilar turns/0.5 mm wire/binocular ferrite R13 : 1.8 KΩ C8 : 47 nf T3 : 2N2222 core 14x8x8 mm/type 43 material R14 : 47 KΩ C9 : 82 pf T4 : 2N2222 R15 : 1.2 KΩ C10 : 33 nf T5 : 2N2222 R16 : 100 KΩ : 4.7 pf T6 : BF324 R17 : 330 Ω C12 : 22 pf T7 : BF244 R18 : 270 Ω C13 : 6.8 pf T8 : 2N2222 R19 : 3.3 KΩ C14 : 68 pf T9 : 2N2222 R20 : 3.3 KΩ C15 : 10 nf U1 : NE602 and the PLL PCB 3

4 Part List of PLL unit R1 : 33 KΩ C2 : 1 nf U6 : CD4050 R2 : 680 Ω C3 : 4.7 nf DZ1 : 12V - ½ W R3 : 47 KΩ C4 : 100 nf X1 : 8 MHz R4 : 330 Ω C5 : 100 µf R5 : 390 Ω C6 : 0.5 µf R6 : 3.3 KΩ C7 : 0.5 µf R7 : 1.2 KΩ C8 : 1.5 µf R8 : 1.8 KΩ C9 : 25 µf R9 : 100 Ω C10 : 33 nf R10 : 5.6 KΩ : 33 nf R11 : 10 KΩ C12 : 33 pf R12 : 10 KΩ C13 : 33 pf R13 : 10 KΩ T1 : 2N2222 R14 : 10 KΩ T2 : 2N2222 R15 : 10 KΩ T3 : 2N2222 R16 : 10 KΩ U1 : 7810 R17 : 10 KΩ U2 : 7805 R18 : 10 KΩ U3 : R19 : 180 KΩ U4 : LM358 C1 : 33 nf U5 : MC this unit may be located anywhere in the rig. The printed boards are double sided fiberglass and components are soldered directly on the copper drawing without drilling, so the lower side can be used to make the ground connections. The ground joints are obtained drilling the board and soldering a wire on both sides. The two PCBs are contained in an aluminium cabinet 70 x 100 x 40 mm. The output RCA sockets and a comb connector for DC supply and band switching are located on a side of the cabinet. Some slides show the overall arrangement. 4

5 How it works I refer to the block diagram and schematic diagrams to describe the functions. supposing to design the VFO in the 5/5.5 Mhz range with a 9 Mhz I.F. in the rig, other frequency values near those indicated can equally match the circuit requirements. The unit described as VFO module is a conversion VFO in the 41 Mhz range. It contains: - a Colpitts VFO ranging from 5 to 5.5 Mhz (the first PCB). The mechanical assembly and the component choice must be very accurate. Use possibly a good variable capacitor (ball bearing supported) and NPO ceramic capacitors. One N150 compensation element contribute to reduce the thermal drift. L1 is wound with 24 turns, 0.8 mm wire on 13 mm plexiglass core to obtain 3.5 µh inductance value. The varactor allows a 20 Khz shift for fine tuning or SPLIT function. N.B. The VFO circuit could be better replaced by a more sophisticated DDS unit like the Digi VFO and related Digi Brain presented in the May 95 and March 96 issues - two buffers wich drive an external frequency counter and the first mixer stage (the second PCB). The output level to the mixer should be about 3 Vpp. This PCB also contains the 7810 power supply. - the first mixer and related 41 Mhz filter (the third PCB). It uses a BF960 mosfet as a mixer and a 2N2222 as a christal driven oscillator to obtain the 36 Mhz output from a 18 Mhz christal. L2 is made by 9 turns, 0.5 mm wire on a T44-2 toroidal core (0.42 µh). A 41 Mhz output filter is obtained by L4 5

6 and L5 (9 turns, 0.5 mm wire on a T44-6 core, 0.34 µh) with a buffer stage (2N2222 transistor). L3 is made with 2 wires wound on L4. The mixer alignement can be made in the following manner : - remove the christal so as the oscillator goes off - input a 41.2 MHz signal to gate 1 and tune the capacitors to obtain maximum output - insert the christal and drive a 5 Mhz signal into gate 1 tuning the 60 pf capacitor for the maximum output (0.7 to 1 Vpp) The VCO unit (forth PCB) contains : - VCO wich covers the range from 12.5 to 39 Mhz using two distinct oscillators switched by a relay driven from an appropriate band switch section. The varactors are high capacity devices for AM use (MVAM115, BB112, etc..). The circuit configuration of the oscillators and use of compensation networks allowed to obtain a good quality and constant level output over the entire frequency range. L1 is wound with 7 turns, 0.8 mm wire on a 5mm plastic support with type 43 ferrite variable core, the inductance range is 0.18 µh (core out) 0.48 µh (core in) L2 is made by 12 turns, 0.5 mm wire on a similar support, the inductance range is 0.52 µh (core out) 1.3 µh (core in). The alignement can be made in the following manner : - supply a 3.5 to 9.5 variable voltage to the varactors (do not excede these limits) - tune the ferrite cores of L6 and L7 to obtain the frequency ranges : 22 to 39 Mhz with L to 22 Mhz with L7 the output level (on a 200 Ohm load) should be about 3 Vpp - second MIXER wich uses an NE602 IC. This device allowed to obtain the best results concerning linearity and balancement over the entire frequency range. The input VCO signal is lowered by a capacitive divider and the two balanced inputs (pins 1 and 2) are driven in opposite phase using a broadband transformer so as to limit the sporious outputs. L3 is made by 5 bifilar 0.5 mm wires into a binocular ferrite core, type 43 material 13x8x8 mm. Some tuning may be required on the value of 2.2 pf capacitor so as to obtain a level of mv pp into pins 1 and 2 of the IC. A buffer stage equipped with two 2N2222 transistors and a compensation network on the second stage emitter allow to obtain a substantially constant output level over the entire frequency range covered by the mixer. This is very important to ensure a good working by the TTL divider. The PLL unit (fifth PCB) contains : - frequency DIVIDER using a TTL 74LS393 wich divides by 64 the frequency coming from mixer. So the output frequency is comparable to the internal reference of the PLL and we can obtain 500 Khz steps (7812,5 Hz x 64 = 500 Khz, see also block diagram). If you have an oscilloscope, you can verify the correct working of the divider stage: - suppressing the connection from the VFO to pin 6 of the NE602 on PCB4 and supplying to the varactors a voltage ranging from 3 to 10 V you should notice no output signal from divider (otherwise try to eliminate the hitch reducing the signal level at pins 1 and 2 of NE602) 6

7 - driving the two varactors in the same manner and reconnecting the VFO you should see a clear TTL signal on both ranges covered (otherwise you can try some change to the compensation network in the buffer stage described above) - PLL circuit using a dedicated Motorola MC This IC features : - a 1024 divider, used to obtain the reference frequency from an 8 Mhz christal - a programmable 9 stages divider, programmed to obtain division ratios from 5 to 57, corresponding to a frequency range from 2.5 to 28.5 Mhz outcoming from the mixer (see also the block diagram). The programming can be done by diode matrix or binary switches according to the following formula (see also the block diagram) : Division Ratio = (32 - desired Mhz band) x 2 where 32 is the difference between the VFO frequency and the IF value (41-9 MHz). To obtain the coverage of the 28,5 Mhz band, for example, you have to set : Division Ratio = (32-28,5) x 2 = 7 a 4050 CMOS Hex Buffer allows to use any programming voltage between 5 and 15 volts, and a Led signals the PLL lock condition - LOOP FILTER using an LM358 operational IC as an integrator, followed by a low pass filter. This circuit showed the best performance concerning to : - locking speed of PLL, so as to follow the VFO frequency changes, also when you are turning quickly the tuning knob - PLL stability - output error voltage clearness, i.e. good spectral purity of the VCO supplied frequency 7

8 The PCBs and main Components layout PCB1 (real dimensions 42 x 68 mm) C13 C9 C14 To C8 L1 C6 T1 out PCB2 C2 R2 C1 split C3 R3 DV1 C7 R4 D1 C5 C4 +10 V R1 PCB2 (real dimensions 42 x 68 mm) C10 R5 T V C12 out PCB3 C20 R10 R6 in PCB1 U1 R8 R7 C15 out display C16 C17 R9 T3 PCB3 (real dimensions 42 x 68 mm) C21 C22 C V L2 L3/L4 C22 R16 R14 C29 X1 R15 C19 T5 R12 R11 C30 C20 in PCB2 C23 T4 R13 L5 C25 C28 C24 T6 C31 R17 C32 out PCB4 C27 8

9 9 PCB4 (real dimensions 67 x 96 mm) T2 T1 L1 C5 R4 T3 RLY R7 R10 T4 out VCO R6 U1 da PCB3 DZ1 R18 R14 C14 L2 T7 T5 T6 C3 C4 C13 C24 R25 T9 to PLL R20 R19 C22 L3 C9 R9 C19 C20 +10V +RLY R24 R11 R12 da LM358 R3 R2 R1 C2 C1 DV1-2 R5 D1 D2 DV3-4 R8 C6 C10 T8 R13 C15 R15 C12 R17 R16 C21 C16 C10 C17 R21 C23 R23 R22 C25 C7 C18 PCB5 (real dimensions 67 x 96 mm) R2 C1 T1 R3 R4 1nF from PCB4 R5 C6 R17 DZ1 R9 U2 U1 +13v R8 R19 C3 X1 C8 R11 R10 C7 C12 LED R7 C4 R1 R15 R12 to VCO U3 T2 C2 U4 T3 R14 R13 R16 R18 U6 U5 C5 C18 R6 C9 C13 5V 10V

10 MC pins layout Final Considerations Making a correct assembly, and following the few suggested rules, the device should work properly without bringing particular troubles. Consider, however, that an adeguate test equipment can make the good result easier. The only hard to find component may be the Motorola MC145106, wich can be ordered to : RF PARTS South Pacific Street - San Marcos - CA